Shoot a pic, focus later

November 09, 2005

Blurry snaps could be a thing of the past with the development of a digital camera that refocuses photos after they have been taken.

The camera could be useful for action shots taken by sports photographers or for CCTV surveillance cameras, which often produce fuzzy shots due to poor lighting.

In an ordinary digital camera, a sensor behind the lens records the light level that hits each pixel on its surface. If the light rays reaching the sensor are not in focus, the image will appear blurry.

Now, Pat Hanrahan and his team at Stanford University have figured out how to adjust the light rays after they have reached the camera. They inserted a sheet of 90,000 lenses, each just 125 micrometres across, between the camera's main lens and the image sensor. The angle of the light rays that strike each microlens is recorded, as well as the amount of light arriving along each ray.

Software can then be used to adjust these values for each microlens to reconstruct what the image would have looked like if it had been properly focused. That also means any part of the image can be refocused - not just the main subject.

Tracing the rays like this removes the conventional trade-off between the aperture size, which controls the amount of light that the camera takes in, and the depth of field. If light is low, a larger aperture will let enough light into the camera to form a clear image, but the laws of optics mean that a narrower slice of the world in front of the camera will appear in focus.

Hanrahan's system would be particularly useful for surveillance cameras, which must work at night but also need to have objects in focus at different distances from the camera.
-end-
Author: Celeste Biever, Boston

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THIS ARTICLE APPEARS IN NEW SCIENTIST MAGAZINE ISSUE: 12 NOVEMBER 2005

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